In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium, to form characters and images. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full colorization, low costs, capability of using ordinary paper as the recording medium, non-contact with printed images and characters, etc. Among such printing methods, in view of enhancing the fastness to light of printed images and characters, an ink-jet printing method utilizing an ink containing a pigment as the colorant has now come to dominate.
In the ink containing a pigment as the colorant, however, the dispersion of the pigment is difficult to often reduce the storage stability of ink. In addition, the gloss of printed images and characters is inferior to those formed by an ink containing a dye as the colorant.
To remove such disadvantages, it has been proposed to use a polymeric dispersant which is made of several kinds of acrylic monomers each having a polyoxyethylene glycol chain (for example, EP 1 323 789A). However, it has been still demanded to develop an ink capable of forming images and characters with more excellent gloss.
There have been also disclosed a pigment composition suitable for ink-jet printing which contains a resin having phosphoric acid groups (for example, JP 2002-201378A), a phosphoric acid-based dispersant and a water-based ink for ink-jet printing containing such a dispersant (for example, JP 2003-506536A). However, the proposed composition and ink are still insufficient in gloss.